Therefore we can't really use it on our old motherboards with pc i express 3 slots right?
Is it likely?
PCIe 4.0 is due out in the next year or two, I believe. 2015 at the latest, but it is no concern. PCIe 3.0 cards are backwards compatible with 2.0. In short, you won't have to change your motherboard, when 4.0 is released.
Also, I think Volta (after Maxwell) will be the first series of Nvidia card to introduce PCIe 4.0
PCIe 3.0's bandwith isn't even fully utilised by current cards I think? So do we really need PCIe 4.0, even with Maxwell and up? If so, that might mean a huge bump in bandwith for these cards which sounds good.
We shouldn't wait until a generation of PCIe becomes saturated. We should have the extra bandwidth, and allow backwards compatability. That way, it is kind to anyone in possession of a 2.0 motherboard, and a 4.0 GPU. If that makes sense? Dual 7970s are very close to saturating 2.0 in x8 x8. So the generations of motherboard and GPU should "intwine" together.